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Atomistic Simulations of the Mechanical Deformation of Irradiation-amorphized Silicon Carbide

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Chinese Journal of Mechanical Engineering 2010年第6期23卷 671-676页

作者： XUE Kun NIU Lisha shi huiji Department of Engineering Mechanics Tsinghua University Beijing 100084 China

Irradiation-induced atomic-scale defects and lattice disorder in Silicon Carbide （SIC） can significantly affect the material＇s mechanical properties. Currently there lacks a unified physical model capable of describing the law in which the properties of SiC scale with the accumulation of defects, especially in terms of the underlying physical mechanism. To develop fundamental models that are capable of describing the various physical properties of SiC as a function of microstructural change, molecular dynamics simulations of uniaxial tension were performed on a series of irradiation-amorphized SiC （a-SiC） samples with a range of imposed chemical disorder, which is defined as the ratio between the number of homonuclear bonds and heteronuclear bonds （x = Nc-c / Nsi-c）. With increasing chemical disorder, significant alternation of mechanical response of a-SiC has been detected in terms of increasingly pronounced plastic flow. Meanwhile relevant mechanical properties, including Young＇s modulus, strength, yield stress and strain, as well as failure strain scale monotonically with chemical disorder while in distinct manners. Specifically slight chemical disorder （x = 0.045） could induce substantial reduction of Young＇s modulus up to -2%, whereas strength basically linearly varies with chemical disorder until x≈0.5 upon which the variations in mechanical properties tend to saturate. Further examination of the evolution of atomic structure of a-SiC reveals a crossover of deformation mechanisms from homogeneous elastic deformation to localized plastic flow, which accounts for the strong chemical disorder dependence of the mechanical properties as well as mechanical responses of amorphous SiC. This crossover is also manifested in switching of fracture mode from brittle failure dominated by lattice instability in the ligaments between topological disordered clusters to nanoductile failure preceded by percolation of nanocavities. Employing chemical disorder to measure the defec

关键词： silicon carbide irradiation-amorphization molecular dynamics modeling tensile test chemical disorder

Comparison of the very high cycle fatigue behaviors of INCONEL 718 with different loading frequencies

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Science China(Physics,Mechanics & Astronomy) 2013年第3期Mechanics & Astronomy) .卷 617-623页

作者： ZHANG YangYang DUAN Zheng shi huiji Key Laboratory of Applied Mechanics Tsinghua UniversityChinese Ministry of Education

In order to clarify the differences of very high cycle fatigue(VHCF) behavior of nickel based superalloy IN718 with different loading frequencies,stress-controlled fatigue tests were carried out by using ultrasonic testing method(20 KHz) and rotary bending testing method(52.5 Hz),both at room temperatures,to establish stress versus cycles to failure(S-N) *** disclosed that cycles to failure at a given stress level increased with an increase of the applied frequency,i.e.,the higher frequency produced an upper shift of the S-N *** analysis suggested that crack initiation and propagation behaviors had large differences:cracks in low-frequency tests preferentially initiated from multiple sources on the specimen surface,while in high-frequency tests,cracks mostly originated from a unique source of subsurface ***,frequency-involved modeling was proposed,based on the damage accumulation theory,which could well illustrate qualitatively those comparisons due to different loading frequencies.

关键词： frequency effect very high cycle fatigue IN718 superalloy

Determination of mixed-mode interfacial fracture toughness for thermal barrier coatings

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Science China(Physics,Mechanics & Astronomy) 2011年第4期54卷 618-624页

作者： DENG HongXia shi huiji YU HuiChen ZHONG Bin Key Laboratory of Applied Mechanics Tsinghua University Chinese Ministry of Education Beijing 100084 China Beijing Institute of Aeronautical Materials Beijing 100095 China

In this work, a test method was developed to determine the interfacial fracture toughness of the air plasma sprayed （APS） thermal barrier coatings （TBCs） over a wide range of mode mixities. For this mixed-mode test method, the analytical expres- sions for the energy release rate and stress intensity factors were derived based on the energy theory and the concept of ＂equi- valence＂. The fidelity of these expressions was affirmed by selected finite element analysis. The experimental results showed that the critical energy release rate increased with the increase of the positive mode mixity, which was mainly due to the increase in contact/friction effect and plastic work dissipation with increasing shear mode loading. Furthermore, an elliptical interfacial failure criterion in terms of the stress intensity factors was proposed. The agreement between the experimental results in the literature and those in our work indicated that our test method and the corresponding analytical solutions can well determine the interfaeial fracture toughness of the TBCs over a wide range of mode mixities.

关键词： thermal barrier coating multilayer mixed-mode interface fracture toughness energy release rate

Molecular dynamics study on the nano-void growth and coalescence at grain boundary

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Science China(Physics,Mechanics & Astronomy) 2012年第1期55卷 86-93页

作者： LIN EnQiang NIU LiSha shi huiji DUAN Zheng Applied Mechanics Laboratory School of Aerospace Tsinghua University Beijing 100084 China

Molecular dynamics simulations using embedded atom method (EAM) potential were performed to study nano-void growth and coalescence at grain boundary in face-centered cubic bicrystal copper. Thin-plate specimens subjected to uniaxial tension strain with one-void and two-void at the centered grain boundary were employed to analyze the effect of specimen size, temperature and applied strain rate on the stress-strain response, incipient yield strength and macroscopic effective Young's modulus. The evolutions of dislocations, twin bands and void shapes under different specimen sizes were also presented. The obtained results show that, regardless of the void numbers, the specimen sizes, temperature, the applied strain rate had significant influence on the void shape evolution, stress-strain curve and incipient yield strength, while negligible effects on the macroscopic effective Young's modulus except for the temperature. Moreover, the voids growth rate along the grain boundary was also found to be associated with the specimen sizes.

关键词： bicrystal void growth void coalescence grain boundary molecular dynamics simulations embedded atom method

In-situ observation and finite element analysis of fretting fatigue crack propagation behavior in 1045 steel

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Chinese Journal of Aeronautics 2021年第11期34卷 131-139页

作者： Qinan HAN Xusheng LEI Yue SU Shaoshi RUI Xianfeng MA Haitao CUI huiji shi Aero-engine Thermal Environment and Structure Key Laboratory of Ministry of Industry and Information Technology College of Energy and Power EngineeringNanjing University of Aeronautics and AstronauticsNanjing 210016China AML Department of Engineering MechanicsTsinghua UniversityBeijing 100084China AMPTL Department of Mechanical EngineeringTsinghua UniversityBeijing 100084China Sino-French Institute of Nuclear Engineering and Technology Sun Yat-Sen UniversityZhuhai 519082China

In this paper fretting fatigue crack behavior in 1045 steel is studied by in-situ observation and finite element ***-situ fretting fatigue experiments are conducted to capture real-time fretting fatigue crack formation and propagation *** fretting fatigue tests under different load conditions are carried out,then the lifetime and fracture surface are *** crack propagation rates under different loading conditions are measured by in-situ *** in-situ observation,crack initiation location and direction are *** element model is used to calculate J-integral which then is applied to fitting with experimental crack growth rate,and establishing crack growth rate *** fitted S-N curve,it turns out that smaller load ratio leads to higher *** initiates slightly below the point equivalent to line contact of the contact surface in different test conditions,and crack direction shows no obvious relationship with load *** established crack growth rate model well agrees with the test results.

关键词： Crack growth rate Fretting fatigue In-situ observation J-integral 1045 steel

TEMPERATURE EFFECT ON LOW-CYCLE FATIGUE BEHAVIOR OF NICKEL-BASED SINGLE CRYSTALLINE SUPERALLOY

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Acta Mechanica Solida Sinica 2008年第4期21卷 289-297页

作者： Xianfeng Ma huiji shi Jialin Gu Zhaoxi Wang Harald Harders Thomas Malow Department of Engineering Mechanics Tsinghua University Beijing 100084 China Department of Material Science Tsinghua University Beijing 100084 China Siemens Power Generation Miilheim an der Ruhr Germany

The low-cycle fatigue （LCF） behavior of a nickel-based single crystal superalloy with [001] orientation was studied at an intermediate temperature of T0℃ and a higher temperature of To ＋ 250℃ under a constant low strain rate of 10^-3 s^-1 in ambient atmosphere. The superalloy exhibited cyclic tension-compression asymmetry which is dependent on the temperature and applied strain amplitude. Analysis on the fracture surfaces showed that the surface and subsurface casting micropores were the major crack initiation sites. Interior Ta-rich carbides were frequently observed in all specimens. Two distinct types of fracture were suggested by fractogaphy. One type was characterized by Mode-I cracking with a microscopically rough surface at To ＋ 250℃. Whereas the other type at lower temperature T0℃ favored either one or several of the octahedral {111} planes, in contrast to the normal Mode-I growth mode typically observed at low loading frequencies （several Hz）. The failure mechanisms for two cracking modes are shearing of γ＇ precipitates together with the matrix at T0℃ and cracking confined in the matrix and the γ/γ＇interface at To - 250℃.

关键词： low cycle fatigue single crystal nickel-based superalloy

Comparison of the very high cycle fatigue behaviors of INCONEL 718 with different loading frequencies

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Science China Chemistry 2013年第3期56卷 617-623页

作者： ZHANG YangYang DUAN Zheng shi huiji Key Laboratory of Applied Mechanics Tsinghua UniversityChinese Ministry of Education

关键词： frequency effect very high cycle fatigue IN718 superalloy

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高温结构材料的蠕变和疲劳研究的一些新进展

固体力学学报 2010年第6期31卷 696-715页

作者：施惠基马显锋于涛清华大学航天航空学院应用力学实验室北京100084

高温部件长期在高温严酷环境和载荷作用下,蠕变和疲劳是制约其使用寿命的重要因素,成为高温结构强度领域的关键问题,近年来日益受到研究者们的高度关注.本文回顾了实际高温结构部件和材料的失效机制、损伤评估以及寿命预测等方面的研究... 详细信息

高温部件长期在高温严酷环境和载荷作用下,蠕变和疲劳是制约其使用寿命的重要因素,成为高温结构强度领域的关键问题,近年来日益受到研究者们的高度关注.本文回顾了实际高温结构部件和材料的失效机制、损伤评估以及寿命预测等方面的研究进展,着重介绍了本课题组在高温蠕变和疲劳领域的最新研究成果,并对该领域今后的发展方向进行了展望.

关键词：蠕变损伤疲劳失效准则镍基单晶合金再结晶

热机械循环载荷下短纤维增强金属基复合材料性能分析

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机械强度 2005年第3期27卷 345-352页

作者：施惠基贾大炜程蕾清华大学工程力学系北京100084

从复合材料内部组分的细观力学关系入手,选取代表体积元,基于Eshelby椭圆夹杂理论和瞬时体积平均的概念,通过集中张量描述纤维与基体以及纤维与纤维间的相互作用,并把在弹性范围内得到的各集中张量推广到弹塑性范围内,建立能够在弹塑性... 详细信息

从复合材料内部组分的细观力学关系入手,选取代表体积元,基于Eshelby椭圆夹杂理论和瞬时体积平均的概念,通过集中张量描述纤维与基体以及纤维与纤维间的相互作用,并把在弹性范围内得到的各集中张量推广到弹塑性范围内,建立能够在弹塑性范围内分析热机械循环载荷作用下短纤维增强金属基复合材料的性质的模型。为了接近工程实际,假设纤维始终保持线弹性,对基体材料采用能反映bauschinger效应的混合硬化模型,依据基体的弹塑性状态决定复合材料整体的弹塑性状态。在塑性范围内,从各向异性的角度出发,采用增量法迭代得出每个加载步结束时复合材料整体以及各相的应力应变增量。编写控制应变和温度加载条件下,计算复合材料应力应变响应的程序,着重讨论纤维的外形、空间分布、体积百分比以及温度载荷对复合材料宏观性质的影响,并与相关的实验结果和数值结果进行比较。

关键词：短纤维增强金属基复合材料热机械循环载荷混合硬化模型弹塑性瞬时体积平均集中张量